Gary Stevens

Thermal-metamorphic signature of an impact event in the Vredefort dome, South Africa

Pseudotachylitic breccias and shock deformation features related to the 2.02 Ga formation of the Vredefort dome by meteorite impact were overprinted by a static metamorphic event, the intensity of which decreased from granulite-facies ( T ≥700 °C) in the center of the dome to greenschist-facies ( T ≤400 °C) around its margins. Geobarometric estimates of 0.2–0.3 GPa for the metamorphic parageneses indicate some 8–11 km of erosion since the impact event. The strong lateral thermal gradient implied by these P-T results is attributed to the combined effects of differential uplift of mid-crustal rocks heated along a pre-impact geotherm and increased shock heating of the target crust toward the center of the impact structure. We suggest that the exceptionally high grade of metamorphism in the center of the dome may, in part, reflect an elevated regional geothermal gradient of ∼25 °C/km in the target crust due to lingering thermal effects related to the 2.05–2.06 Ga Bushveld magmatic event.

Mid-crustal granulite facies metamorphism in the Central Kaapvaal craton: the Bushveld Complex connection

Abstract The numerous greenstone remnants which occur as inclusions in the Archaean gneissic granitoid basement exposed in the core of the Vredefort Dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three-stage metamorphic history: (1) in metapelites, migmatites and refractory restites were produced in a mid-crustal (±0.5 GPa) M1 anatectic event, at temperatures in excess of 900°C (in the restites); (2) in the migmatites, the peak metamorphic assemblages are variably overprinted by a high-grade retrograde reaction which involved crystallizing anatectic melts (M2); (3) both the peak metamorphic assemblages and the retrogression textures are cross-cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the Vredefort Dome; and (4) the peak metamorphic assemblages and the M2 retrogression textures are overprinted by new generations of remarkably fine-grained, post-shock, high-grade phases which crystallised at a pressure some 0.25 GPa lower than that of both the peak metamorphic conditions and the retrogression (M3). These data indicate an anticlockwise P-T evolution where mid-crustal anatexis most probably resulted from the intraplating of Bushveld Complex-related ultramafic magmas at 2.06 Ga. A period of approximately isobaric cooling and retrogression was followed by the Vredefort Catastrophe, which resulted from the impact of a large meteorite into the terrane at 2.02 Ga. This shock event halted the natural mid-crustal metamorphic progression prior to the terrane cooling below 650°C, and resulted in the rapid exhumation of the terrane by some 9 km. In the process, it provided a window into the deep levels of the central Kaapvaal craton not seen elsewhere in the region.

The effects of temperature and pressure on gold–chloride speciation in hydrothermal fluids: a Raman spectroscopic study

Raman spectra have been obtained for gold–chloride solutions at elevated temperature and pressure, with gold concentrations of 0.005 to 0.04 M and varying pH. A hydrothermal cell has been developed that allows Raman analyses up to 300°C and 2 kbar. Using simple Au (III) chloride solutions at low pH and up to 300°C, the expected transformation to Au (I) with increasing temperature was not found, even in solutions prepared under reducing conditions. The Au (III) complex [AuCl4]− breaks down and precipitates gold at temperatures above 250°C, but the exact temperature appears to be related to oxygen fugacity. The Raman spectra showed consistent trends in band parameters with increasing temperature, indicating minor changes in bond lengths but no change in speciation. In the range investigated, pressure had a minimal effect on both speciation and band frequencies. However, on heating the Au (III) complex [AuCl4]− in the presence of metallic gold, transformation to the Au (I) form [AuCl2]− does occur. The single Raman band for the complex [AuCl2]− was recorded at ≈326/cm at 250°C. A previously described band for this species was probably an artifact of the peak fitting process. The importance of comparing trends in band parameters to determine optimal peak fitting is highlighted. Solutions at pH values of 5.75 and 6.5, which had different mixed chloro–hydroxy complexes at ambient temperature, showed changes in speciation with increasing temperature. In the lower pH solution (OH)− groups were replaced by Cl− ligands on heating, resulting in a transformation from [AuCl3(OH)]− to [AuCl4]−. In the higher pH solution, there was initially an increase in the number of (OH)− ligands, from [AuCl2(OH)2]− to a mixture with [AuCl(OH)3]− at 75 to 100°C, but with increasing temperature, this trend was reversed and [AuCl4]− became dominant. No effect of pressure on the chloro–hydroxy speciation was observed.

Hydrous cordierite in granulites and crustal magma production

Huge volumes of S-type granitic and volcanic rocks, as well as many migmatites, are the products of fluid-absent partial melting in metasedimentary protoliths at granulite-facies conditions. Thus, it is essential to know the controls on the fertility of metasedimentary rocks as magma sources. In the present experiments on fluid-absent partial melting of biotite in metasedimentary rocks, cordierite proportion is inversely correlated with melt fraction. At constant pressure ( P ) and temperature ( T ), this variation in melt proportion is a function of bulk-rock Mg number [100 Mg/(Mg + Fe)] and activity of alumina ((alpha)Al(subscript)2O(subscript)3). However, melt compositions are essentially unaffected. At 0.5 GPa, the calculated H2O contents of cordierite decrease as T increases from 850 to 1000 ° C. In aluminous metasedimentary rocks, the cordierite stability field considerably overlaps with the conditions of fluid-absent melting in the middle to lower crust, severely restricting melt productivity. Consequently, metapelites may be much less fertile sources of granitic magma than metagraywackes.